You may need to format the drive using the
Disk Management utility.
Try this:

In the Windows Explorer, go to (and click the + next to)
Control Panel, then press on
Administrative Tools.

Double click
Computer Management and wait for the new window to open.

In the new window, on the left hand side go down to
Storage and then click on
Disk Management.

In the right hand window, you should see you new drive.
Right-Click on the external hard drive and select
New Partition.

Follow the New Partition Wizard, ensuring that you accept the defaults. The only options you will really want to change are the
Drive Letter and the
Drive Name.

Once you finish the Wizrd, it will format the drive. You can then close the Computer Management window, and go back to your Windows Explorer. Your new drive should be in there now!!
Hope that helped!!!